1. Field of the Invention
The present invention relates to a steering column of a power steering apparatus.
2. Description of the Invention
In general, a power steering apparatus is used for satisfying consumers demand to carry out light, swift steering operations. As shown in FIG. 1, the manipulation force generated by the steering wheel 2 is transmitted through a steering column 4 to a gear box 6 and further transmitted through steering linkage including a pitman arm 8 to the front wheels. At this time, the hydraulic pressure generated by driving an oil pump 10 decreases the manipulation force of the steering wheel 2. An oil pump 10 supplies oil from an oil tank 12 through a hydraulic line.
As shown in FIG. 2, inside a housing 14 of the gear box 6, large and small torsion bars 16 with two different spring constants are integrally installed at the end of steering shaft 15 of the steering column 4, and a control valve with two pairs of flappers 18, 18xe2x80x2 is installed to utilize the twist of the torsion bars 16 made by the manipulation force of the steering wheel 2 and the friction between the front wheels and earth to change the hydraulic route by the flappers.
Accordingly, the power steering apparatus thus constructed has a pressure-torque characteristic determined by the thickness of the torsion bars 16, the end shape and size of the steering shaft 15.
Generally, if the torsion bars 16 are thin and if the characteristic determined by the end shape and size of the steering shaft 15 is light, a weak manipulation force may manipulate the steering wheel but cause a deterioration in its on-center feeling. In other words, the steering wheel may be rotated by a slight force but may create a problem of instability in the steering force when a vehicle is driven at a high speed.
In addition, if the torsion bars are thick and the characteristic of the end of the steering shaft 15 is heavy, there may be another problem of requiring greater manipulation force in spite of its greater on-center feeling.
Furthermore, if the pressure-torque characteristic is made light and if thin torsion bars are used to achieve a slight manipulation force, the steering wheel is sensitively reacted to external turbulences transmitted from tires of a driven car to thereby cause a problem of losing stability in operations.
It is an object of the present invention to solve the aforementioned problems and provide a steering column of a power steering apparatus which can improve on-centering feeling while a car is driven at a high speed, and can make its manipulation force lighter while the car is driven at a low speed or parked for its complete stop.
In order to accomplish the aforementioned object of the present invention, there is provided a steering column of a power steering apparatus constructed with a tube fixed at the steering column and a steering shaft inserted at the inner side of the tube to transmit the manipulation force to steering linkage according to manipulation of a steering wheel. The steering column comprises:
a screw part formed at the external surface of the steering shaft;
guide rails installed at the internal side of the tube correspondingly to the screw part; and
a ring coupled with a screw of the screw part for moving lengthwise along the guide rail to the steering shaft, where protruded parts are respectively formed at the lengthwise center of the guide rail and the ring. It is preferable that the guide rail and the ring are made of a plastic material with a predetermined degree of elasticity.
It is preferable that stoppers are fastened at both sides of the guide rail to restrict the ring in the distance of its motion.